KEYWORDS: 3D optical data storage, Super resolution, 3D image processing, Objectives, Photonic crystals, Data storage, Absorption, Microfabrication, 3D microstructuring, Multiphoton processes
Different types of objective lens (OL), i.e. circular, annular and one-dark-ring, are proposed and demonstrated to control the aspect ratio (AR) of micro-focus-region of high numerical aperture (NA) OL. Namely, the AR decreases from about 7.3 to 2.7 in case of using a circular OL with NA changes from 0.7 to 1.4, respectively. By using an annular OL, the transverse size of the focal spot of micro-focus-region decreases but its longitudinal size increases, so that the AR increases several times with respect to the case of circular OL. In particular, when using the one-dark-ring OL, one can decrease both transverse and longitudinal sizes of the focal spot or decrease only the longitudinal size, so that the AR obtained with a one-dark-ring OL is decreased to about 70% of that obtained with a circular OL. Such lenses can be useful for many applications such as sub-microfabrication and three-dimensional data storage using multi-photon absorption process.
A simple optical interference method for fabricating two- and three-dimensional (2D and 3D) periodical structures is theoretically and experimentally demonstrated. Multiple-exposure of two-beam interference pattern into a photopolymerizable resist creates high quality 2D or 3D microstructures. The type of periodic structure depends on the orientation of the photoresist with respect to the laser beams and the number of exposure. Square or hexagonal structures are obtained by choosing an angle of 90° or 60°, respectively, between two different exposures. 2D structures are obtained with two or three equal exposures. 3D structures with different types (bcc, fcc, Woodpile, etc.) are obtained with three or four exposures and appropriate rotation angles. This method presents many advantages over others using multi-beam (three-, four-, or five-beam) interference: i) easy to fabricate different structures(hexagonal or square) by simply rotating the sample, ii) best contrast between the minimal and maximal intensities of interference pattern due to the identical polarization of two laser beams in the interference area, iii) in particular, 3D periodical structures have the same period in three dimensions, which can't be obtained by one exposure of multi-beam interference. The experimental results obtained with SU-8 negative photoresist are well in agreement with the theoretical predictions. Such fabrication technique can be useful for applications in photonic crystals research.
We demonstrate a promising method to fabricate large-area photonic crystals with desired defects by using the combination of interference and multi-photon polymerization techniques. Multiple-exposure of two-beam interference pattern at 325 nm into a negative SU-8 photopolymerizable photoresist is used to form a square or hexagonal twodimensional periodic structure. Arbitrary defects are introduced in these structures by tightly focused (numerical aperture 0.85) 100 fs duration pulses at 830 nm to generate multi-photon polymerization effect. The experimental evidence of 6 mm × 6 mm photonic crystals with the lattice constant as small as 1 μm embedding several kinds of defect proves the concept and shows this technique potentially useful for photonic researches and applications.
Photoreactive third-harmonic (TH) generation at 355 nm in diarylethene- polymethylmethacrylate (DE-PMMA) polymer thin films is obtained by either one- or two-photon excitation. TH intensity generated from a DE-PMMA polymer thin film decreases, when it is pumped by either 325 nm or 442 nm, which change molecular structure of DE molecules from open-form (A form) to closed-form (B form). TH intensity recovers to its original intensity level, after all B form DE molecules return to A form induced by 532 nm or 1064 nm laser irradiation. The experimental results reveal that the second-order hyperpolarizability (γ) of A form molecules may be larger than that of B form molecules. Moreover, TH output efficiency is independent of the angle between the pump and probe polarization directions. Those experimental results were explained by using a photoinduced isomerization theory based angular hole burning and angular redistribution mechanisms for two-dimensional structure of DE molecules including the two-photon absorption effect of B form. The simulation results are consistent with those of optical pumping TH experiment.
Temperature effect on photoinduced third harmonic generation (THG) variation of an azo copolymer and an azo guest-host polymer is studied at two different temperatures. At higher temperature, both angular hole burning (AHB) and molecule angular redistribution (AR) motions are smaller due to the decrease of cis-trans thermal relaxation time and cis population. Smaller photoinduced THG change is observed in both samples at higher temperature. THG recovery experiment results show copolymer thin films pumped at high temperature have the best photoinduced THG variation stability after turning off the pump beam.
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